Subtraction by Addition: CHECK! Sometimes I hate my car - Page 8 - The Unofficial BMW M5 Messageboard (m5board.com)

jrhaile · 8th July 2009, 18:40

Thanks Chuck for the heads up

You're a true value to this community. And I hit the 7400RPM Rev Limited on accident

churious · 8th July 2009, 19:10

So... I should start seeing a gain on my 3.45 diff once I do header and race cats in a few weeks. According to CSBM5....???

Sorry to jack your thread james...

CSBM5 · 8th July 2009, 19:57

Quote:

Originally Posted by churious

So... I should start seeing a gain on my 3.45 diff once I do header and race cats in a few weeks. According to CSBM5....???

Sorry to jack your thread james...

I would venture to guess that you will not significantly move the torque peak upward unless you uncork the intake side ala Dinan S2 (i.e. larger MAFs, larger throttle bodies and perhaps most important matched and larger intake horns which resonate at a higher rpm). You'll gain from high-end HP from the headers assuming your tune can handle it, and if they're 4-1 headers, you'll probably like the fact you have a 3.45 afterwards since they generally give up low to midrange torque compared to stock.

My point was that Dinan used the 3.45 to match the S2's higher torque and HP rpm peaks. It wasn't that the 3.15 is deficient in some way just that when you mod the motor's volumetric efficiency peak (torque peak), you need to adjust gearing to match the new powerband characteristics. Hence in that case, it's helpful, although not necessary, to have the 3.45...especially since they raise the redline too.

Note to those raising the S62 rev limit above 7k rpms. You need to be very proactive in worrying about/replacing your rod bearings. My personal opinion is that revving the S62 to above 7k rpm is just not worth the exponential risk increase to the rod bearings for what little is achieved in acceleration. The risk-reward is vastly weighted toward risk with very little reward.

jrhaile · 8th July 2009, 20:02

Quote:

Originally Posted by churious

So... I should start seeing a gain on my 3.45 diff once I do header and race cats in a few weeks. According to CSBM5....???

Sorry to jack your thread james...

No worries... it appears more evident that your car needs the help

xr4tic · 8th July 2009, 20:13

So I was playing with excel, and plugged in the torque vs. RPM and factored in gearing.

If you're doing a 3rd gear pull from 3000 RPM, then the 3.45 gears will be better until about 5000RPM, and then the stock gears are better above that, because the Torque falls off so drastically.

This is probably why everyone says the 3.45 gears feel so good when driving around town, but in an actual race, when you spend all your time in the upper RPM range, they come up short.

Look at your dyno plot:

Here's the math, 3.15 vs. 3.45, you will get 10% more torque, but at 10% higher RPM for the same speed
3.15 gear @3000 RPM - 325 ft/lbs
3.45 gear @3300 RPM - 330 ft/lbs x 10% = 363 ft/lbs

At that point, you're putting down 11% more torque at the same speed

Now lets look at the upper RPM range
3.15 gear @ 6000 RPM - 280 ft/lbs
3.45 gear @ 6600 RPM - 250 ft/lbs x 10% = 275 ft/lbs
You are now making 2% LESS torque at that speed.

3.15 gear @ 6400 RPM - 265 ft/lbs
3.45 gear @ 7040 RPM - 220 ft/lbs x 10% = 242 ft/lbs
10% less ft/lbs here, and now you have to shift, where the stock gears have another 600 RPM to go, giving you an even greater advantage

gsfent · 8th July 2009, 22:43

Quote:

Originally Posted by xr4tic

So I was playing with excel, and plugged in the torque vs. RPM and factored in gearing.

If you're doing a 3rd gear pull from 3000 RPM, then the 3.45 gears will be better until about 5000RPM, and then the stock gears are better above that, because the Torque falls off so drastically.

This is probably why everyone says the 3.45 gears feel so good when driving around town, but in an actual race, when you spend all your time in the upper RPM range, they come up short.

Look at your dyno plot:

Here's the math, 3.15 vs. 3.45, you will get 10% more torque, but at 10% higher RPM for the same speed
3.15 gear @3000 RPM - 325 ft/lbs
3.45 gear @3300 RPM - 330 ft/lbs x 10% = 363 ft/lbs

At that point, you're putting down 11% more torque at the same speed

Now lets look at the upper RPM range
3.15 gear @ 6000 RPM - 280 ft/lbs
3.45 gear @ 6600 RPM - 250 ft/lbs x 10% = 275 ft/lbs
You are now making 2% LESS torque at that speed.

3.15 gear @ 6400 RPM - 265 ft/lbs
3.45 gear @ 7040 RPM - 220 ft/lbs x 10% = 242 ft/lbs
10% less ft/lbs here, and now you have to shift, where the stock gears have another 600 RPM to go, giving you an even greater advantage

I see your logic and seems to make a lot of sense. But I am not sure how you worked out the math; to me it looks like you are double counting the 3.45's 10% torque increase.

I assume you have equalized road speed in gear, so at any given speed, the 3.45 is at roughly 10% higher rpm. Isn't that the 10% increase due to gearing?

You then multiply again by 10%, presumably for the extra torque? I could see that at the same rpm, but not at the same road speed. What did I miss?

Regards,
Jerry

E55AMG2 · 8th July 2009, 22:48

I agree with Chuck on the shorter (steeper) gearing. Everyone seems to be dancing around the point...the only times when it does come in handy are;

1) when you have a high redline (or extended rev limiter) to take advantage of the additional thrust to the wheels. Examples of this are the E60 M5, E90/2 M3, Ferrari Enzo (as was quoted in the other thread), F430...blah blah blah. Basically cars that rev very very high and do not make a lot of low end torque. Dinan offers their 3.45 diff with the S2 and S3 packages simply because of the extended redline.

2) when you have an automatic transmission with few ratios that are widely spaced. The short rear end is no problem due to the fact that the gears in the trans are so tall and widely spaced. This also goes for manual transmissions with the same setup.

Otherwise, the extra thrust is offset by the additional shifts necessary to keep up.

2k1blkm5 · 8th July 2009, 23:24

I am still not convinced that 2.65 vs. 3.64 gearing and there's no change. At some point, the time it takes to make the extra shift is more than you gained from accelerating faster, thus it does not make a big difference. However, when the time you gained by acclerating faster is greater than the time wasted on the extra shift, the lower gearing clearly helps.

To see such effect amplified and more clearly, we need to look at the extreme ends of each case. Imagine that one car has super high gearing that it takes only 1 shift to get to 100mph. The second car has lower gearing and needs 2 shifts to get to 100mph. (Or one car has a 1.5:1 diff and the second car has 3.5:1 diff if you will) Now, the second car runs through the gears faster, by 40mph it has shifted into 2nd gear. The first car is still in first gear, it does not have to shift till 60mph. By theory, once the second car shifted into 2nd gear, even when it's ahead, the first car should catch up, until it shifts. But the fact is that the second car would have out accelerated the first car by so much that the first car would never catch up (until it runs out of gears or top end speed). Unless someone thinks both cars will get to 100mph equally quick??

Someone please put in 1.5:1 and 3.5:1 into the the excel sheet and see what results we get.

I know there are other factors such as engine rev limit etc. You can point to a Ferrari and what not, but who can honestly look at a engine and its curve and state we need a 2.87 final gearing (or whatever) to get the max performance out of that engine? Let's keep those a constant and just examine the effects of different gearing, since that's what we are concerned with now.

jrhaile · 8th July 2009, 23:27

This thread has become too complicated for me to understand LOL! Lots of smart number people on m5board

CSBM5 · 8th July 2009, 23:31

Quote:

Originally Posted by 2k1blkm5

I am still not convinced that 2.65 vs. 3.64 gearing and there's no change. At some point, the time it takes to make the extra shift is more than you gained from accelerating faster, thus it does not make a big difference. However, when the time you gained by acclerating faster is greater than the time wasted on the extra shift, the lower gearing clearly helps.

The goal is balance not extremes. Why not put a 6:1 rear gear in the car?

You'll get a ton of torque multiplication at the wheels...car will be massively faster, right?

Read Lscman's post.